Chromosome abnormalities are associated with genetic disorders, degenerative diseases, and exposure to agents known to cause degenerative diseases, particularly cancer, German, “Studying Human Chromosomes Today,” American Scientist, 58: 182–201 (1970); Yunis; “The Chromosomal Basis of Human Neoplasia,” Science, 221: 227–236 (1983); and German, “Clinical Implication of Chromosome Breakage,” in Genetic Damage in Man Caused by Environmental Agents, Berg, Ed., pgs. 65–86 (Academic Press, New York, 1979). Chromosomal abnormalities can be of several types, including: extra or missing individual chromosomes, extra or missing portions of a chromosome (segmental duplications or deletions), breaks, rings and chromosomal rearrangements, among others. Chromosomal or genetic rearrangements include translations (transfer of a piece from one chromosome onto another chromosome), dicentrics (chromosomes with two centromeres), inversions (reversal in polarity of a chromosomal segment), insertions, amplifications, and deletions.
Detectable chromosomal abnormalities occur with a frequency of one in every 250 human births. Abnormalities that involve deletions or additions of chromosomal material alter the gene balance of an organism and generally lead to fetal death or to serious mental and physical defects. Down syndrome can be caused by having three copies of chromosome 21 instead of the normal 2. This syndrome is an example of a condition caused by abnormal chromosome number, or aneuploidy. Down syndrome can also be caused by a segmental duplication of a subregion on chromosome 21 (such as, 21q22), which can be present on chromosome 21 or on another chromosome. Edward syndrome (18+), Patau syndrome (13+), Turner syndrome (XO) and Kleinfelter syndrome (XXY) are among the most common numerical aberrations. [Epstein, The Consequences of Chromosome Imbalance: Principles, Mechanisms and Models (Cambridge Univ. Press 1986); Jacobs, Am. J. Epidemiol, 105: 180 (1977); and Lubs et al., Science, 169: 495 (1970).]
Retinoblastoma (del 13q14), Prader-Willis syndrome (del 15q11–q13), Wilm's tumor (del 11p13) and Cri-du-chat syndrome (del 5p) are examples of important disease linked structural aberrations. [Nora and Fraser, Medical Genetics: Principles and Practice, (Lea and Febiger (1989).]
One of the critical endeavors in human medical research is the discovery of genetic abnormalities that are central to adverse health consequences. In many cases, clues to the location of specific genes and/or critical diagnostic markers come from identification of portions of the genome that are present at abnormal copy numbers. For example, in prenatal diagnosis, as indicated above, extra or missing copies of whole chromosomes are the most frequently occurring genetic lesion. In cancer, deletion or multiplication of copies of whole chromosomes or chromosomal segments, and higher level amplifications of specific regions of the genome, are common occurrences.
Much of such cytogenetic information has come over the last several decades from studies of chromosomes with light microscopy. For the past thirty years cytogeneticists have studied chromosomes in malignant cells to determine sites of recurrent abnormality to glean hints to the location of critical genes. Even though cytogenetic resolution is limited to several megabases by the complex packing of DNA into the chromosomes, this effort has yielded crucial information. Among the strengths of such traditional cytogenetics is the ability to give an overview of an entire genome at one time, permitting recognition of structural abnormalities such as inversions and translocations, as well as deletions, multiplications, and amplifications of whole chromosomes or portions thereof. With the coming of cloning and detailed molecular analysis, recurrent translocation sites have been recognized as involved in the formation of chimeric genes such as the BCR-ABL fusion in chronic myelogeneous leukemia (CML); deletions have been recognized as frequently indicating the location of tumor suppressor genes; and amplifications have been recognized as indicating overexpressed genes.
Conventional procedures for genetic screening and biological dosimetry involve the analysis of karyotypes. A karyotype is the particular chromosome complement of an individual or of a related group of individuals, as defined both by the number and morphology of the chromosomes usually in mitotic metaphase. It include such things as total chromosome number, copy number of individual chromosome types (e.g., the number of copies of chromosome X), and chromosomal morphology, e.g., as measured by length, centromeric index, connectedness, or the like. Karyotypes are conventionally determined by chemically staining an organism's metaphase, prophase or otherwise condensed (for example, by premature chromosome condensation) chromosomes. Condensed chromosomes are used because, until recently, it has not been possible to visualize interphase chromosomes due to their dispersed condition and the lack of visible boundaries between them in the cell nucleus.
A number of cytological techniques based upon chemical stains have been developed which produce longitudinal patterns on condensed chromosomes, generally referred to as bands. The banding pattern of each chromosome within an organism usually permits unambiguous identification of each chromosome type (Latt, “Optical studies of Metaphase Chromosome Organization,” Annual Review of Biophysics and Bioengineering, 5: 1–37 (1976)].
Unfortunately, such conventional banding analysis requires cell culturing and preparation of high quality metaphase spreads, which is time consuming and labor intensive, and frequently difficult or impossible. For example, cells from many tumor types are difficult to culture, and it is not clear that the cultured cells are representative of the original tumor cell population. Fetal cells capable of being cultured need to be cultured for several weeks to obtain enough metaphase cells for analysis. Over the past decade, methods of in situ hybridization have been developed that permit analysis of intact cell nuclei-interphase cytogenetics. Probes for chromosome centromeres, whole chromosomes, and chromosomal segments down to the size of genes, have been developed. With the use of such probes, the presence or absence of specific abnormalities can be very efficiently determined; however, it is tedious to test for numerous possible abnormalities or to survey to discover new regions of the genome that are altered in a disease.
The present invention, Comparative Genomic Hybridization (CGH) [formerly called Copy Ratio Reverse Cytogenetics (CRRC) among other names] provides powerful methods to overcome many of the limitations of existing cytogenetic techniques. When CGH is applied, for example, in the fields of tumor cytogenetics and prenatal diagnosis, it provides methods to determine whether there are abnormal copy numbers of nucleic acid sequences anywhere in the genome of a subject tumor cell or fetal cell or the genomes from representative cells from a tumor cell population or from a number of fetal cells, without having to prepare condensed chromosome spreads from those cells. Thus, cytogenetic abnormalities involving abnormal copy numbers of nucleic acid sequences, specifically amplifications and/or deletions, can be found by the methods of this invention in the format of an immediate overview of an entire genome or portions thereof. More specifically, CGH provides methods to compare and map the frequency of nucleic acid sequences from one or more subject genomes or portions thereof in relation to a reference genome. It permits the determination of the relative number of copies of nucleic acid sequences in one or more subject genomes (for example, those of tumor cells) as a function of the location of those sequences in a reference genome (for example, that of a normal human cell).
Gene amplification is one of several mechanisms whereby cells can change phenotypic expression when increased amounts of specific proteins are required, for example, during development [Spradling and Mahowald, PNAS (USA), 77: 1096–1100 (1980); Glover et al., PNAS (USA 79: 2947–2951 (1982), or during an environmental challenge when increased amounts of specific proteins can impart resistance to cytotoxic agents [Melera et al., J. Biol. Chem. 255: 7024–7028 (1980); Beach and Palmiter, PNAS (USA, 78: 21102114 (1981)].
A major limitation of Southern analysis and related conventional techniques for analysis of gene amplification is that only specific sites are studied leaving the vast majority of the genome unexamined. Conventional cytogenetic studies, on the other hand, provide a broad survey of the genome but provide little information about genes that may be involved in amplification events. However, the procedures of this invention overcome those limitations. This invention can be used to show the normal chromosomal locations of all regions of a genome that are amplified or deleted wherein the size of the regions that can be detected is limited only by the resolution of the microscopy used and the organization of DNA in condensed chromosomes. Thus, this invention provides among other uses the ability to study gene amplifications and deletions and their roles in tumor development, progression and response to therapy more thoroughly than was possible previously. The methods of CGH are sufficiently rapid and simple that large numbers of subject nucleic acids, for example from many tumors, can be analysed in studies for gene amplification and deletion.
The karyotypic heterogeneity in solid tumors can be extreme. Identification of commonly occurring chromosomal changes by analysis of metaphase spreads is often difficult or impossible using conventional banding analysis because of the complexity of the rearrangements and because of the poor quality of the metaphase preparations. CGH overcomes that limitation in that the tumor nucleic acid can be studied without the requirement of preparing metaphase spreads. Since CGH can probably be performed on single cells by amplifying the nucleic acid therefrom, CGH can be used to investigate the heterogeneity of tumors by studying representative cells from different cell populations of the tumor. Alternatively, CGH of nucleic acid from a tumor extracted in a bulk extraction process from many cells of the tumor can reveal consistencies within the apparent heterogeneity. For example, the same amplified sequences may appear as homogeneously staining regions (HSRS) and/or double minute chromosomes (DMs) in one tumor cell but as an extension of a chromosome arm in another tumor cell. Thus, order from the apparent randomness may be realized by CGH hybridization.
Montgomery et al., PNAS (USA), 80: 5724–5728 (September 1983), concerns the hybridization of labeled Cot fractionated DNAs from tumor cell lines (a Cot fraction from which the high copy repeats, low copy repeats and single copy sequences were substantially removed) to metaphase spreads from said tumor cell lines. Basically, Montgomery et al. mapped the positions of nucleic acid sequences from tumor cell lines that are very highly amplified back to tumor cell line genomes.
Total genomic DNA from one species has been used in in situ hybridization to discriminate in hybrid cells between chromosomes of that species and of a different species on the basis of the signal from the high copy repetitive sequences. [Pinkel et al., PNAS (USA), 83: 2934 (1986); Manuelidis, Hum. Genet., 71: 288 (1985); and Durnam et al., Somatic Cell Molec. Genet., 11: 571 (1985).] Landegent et al., Hum. Genet., 77: 366–370 (1987), eliminated highly repetitive sequences, like Alu and Kpn fragments, from whole cosmid cloned genomic sequences by blocking the highly repetitive sequences with Cot-1 DNA. The resulting probe was used for in situ hybridization.
European Patent Application Publication No. 430,402 (published Jun. 5, 1991) describes methods and compositions for chromosome-specific painting, that is, methods and compositions for staining chromosomes based upon nucleic acid sequence employing high complexity nucleic acid probes. In general in the chromosome-specific painting methods, repetitive sequences not specific to the targeted nucleic acid sequences are removed from the hybridization mixture and/or their hybridization capacity disabled, often by blocking with unlabeled genomic DNA or with DNA enriched for high copy repetitive sequences as is Cot-1 [commercially available from Bethesda Research Laboratory, Gaithersburg, Md. (USA)]. Pinkel et al., PNAS (USA), 85: 9138–9142 (1980) also describes aspects of chromosome-specific painting as well as International Publication No. WO 90/05789 (published May 31, 1990 entitled “in situ Suppression Hybridization and Uses Therefor”).
Chromosome-specific repeat sequence probes and chromosome-specific painting probes can be hybridized in e to interphase nuclei as well as metaphase spreads and provide information about the genetic state of the individual targeted genomes. A limitation of such hybridizations is that cytogeneic information is only provided from the regions to which the probes bind. Such hybridizations are very useful for determining if a particular abnormality is present, for example, the deletion of a specific gene or a duplication among other abnormalities, but it is laborious to search for currently unknown abnormalities on a region by region basis.
Other methods of searching for unknown genetic abnormalities similarly require a lot of work. For example, looking for loss of heterozygosity in tumor cells, requires the hybridization of many probes to Southern blots of tumor and normal cell DNA. The instant invention, Comparative Genomic Hybridization (CGH), provides methods to overcome many of the limitations of the existing cytogenetic techniques.
Saint-Ruf et al., Genes, Chromosomes & Cancer, 2: 18–26 (1990) state at page 24 that                Human breast carcinomas are characterized by two sets of molecular anomalies. Firstly, some protooncogenes, such as MYC, INT2, HST, and ERBB2, are frequently found either amplified or overexpressed . . . . Secondly, loss of heterozygosity has been reported, especially for ip, 11, 13 and 17 . . .        Human breast carcinomas are also characterized cytogenetically by various anomalies that may be the chromosomal counterpart of the molecular anomalies: regions of amplification (HSRS) are found in more than one-third of the tumors . . . and various deletions, affecting, e.g., 1p, 11p, 11q, 13, and 17p, are found recurrently . . . .[Citations omitted.] Saint-Ruf et al. concluded from the reported experiments that although amplification of genetic material is a frequent and probably important event in breast carcinogenesis, that the relevant genes involved in such amplifications remain unknown but do not seem to correspond to the proto-oncogenes commonly considered important in breast cancer.        
Since HSRs in tumors are most often not at the site of the amplified gene(s) in normal cells, standard cytogenetics does not yield any information that could assist with identification of the gene(s). CGH on the other hand permits mapping them in the normal genome, a major step towards their identification.
Dutrillaux et al., Cancer Genet. Cytogenet., 49: 203–217 (1990) report (at page 203) that “[a]lthough human breast carcinomas are among the most frequent malignant tumors, cytogenetic data remain scarce, probably because of their great variability and of the frequent difficulty of their analysis.” In their study of “30 cases with relatively simple karyotypes to determine which anomalies occur the most frequently and, in particular, early during tumor progression” (p. 203), they concluded that “trisomy 1q and monosomy 16q are early chromosomal changes in breast cancer, whereas other deletions and gain of 8q are clearly secondary events.” [Abstract, p. 203.] Dutrillaux et al. further state (at page 216) that deletions within tumor suppressor genes “characterize tumor progression of breast cancer.”
It is believed that many solid tumors, such as breast cancer, progress from initiation to metastasis through the accumulation of several genetic aberrations. [Smith et al., Breast Cancer Res. Treat., 18 Suppl. 1: S 514 (1991); van de Vijver and Nusse, Biochim. Biophys. Acta, 1072: 33–50 (1991); Sato et al., Cancer Res., 50: 71847189 (1990).] Such genetic aberrations, as they accumulate, may confer proliferative advantages, genetic instability and the attendant ability to evolve drug resistance rapidly, and enhanced angiogenesis, proteolysis and metastasis. The genetic aberrations may affect either recessive “tumor suppressor genes” or dominantly acting oncogenes. Deletions and recombination leading to loss of heterozygosity (LOH) are believed to play a major role in tumor progression by uncovering mutated tumor suppressor alleles.
Dominantly acting genes associated with human solid tumors typically exert their effect by overexpression or altered expression. Gene amplification is a common mechanism leading to upregulation of gene expression. [Stark et al., Cell, 75: 901–908 (1989).] Evidence from cytogenetic studies indicates that significant amplification occurs in over 50% of human breast cancers. [Saint-Ruf et al., supra.] A variety of oncogenes have been found to be amplified in human malignancies. Examples of the amplification of cellular oncogenes in human tumors is shown in Table 1 below.
TABLE 1AmplifiedDegree ofDM or HSRGeneTumorAmplificationPresentc-mycPromyelocytic leukemia20x+cell line, HL60Small-cell lung 5–30x?carcinoma cell linesN-mycPrimary neuroblastomas 5–1000x+(stages III and IV)and neuroblastomacell linesRetinoblastoma cell10–200x+line and primarytumorsSmall-cell lung carcinoma50x+cell lines and tumorsL-mycSmall-cell lung carcinoma10–20x?cell lines and tumorsc-mybAcute myeloid leukemia 5–10x?Colon carcinoma cell lines10x?c-erbbEpidermoid carcinoma cell30x?Primary gliomas?c-K-ras-2Primarycarcinomas of lung, 4–20x?colon, bladder, and rectumN-rasMammary carcinoma cell 5–10x?lineSOURCE: modified from Varmus, Ann. Rev. Genetics, 18: 553–612 (1984) [cited in Watson et al., MoleculaL Biology of the Gene (4th ed.; Benjamin/Cummings Publishing Co. 1987)]
Chromosomal deletions involving tumor suppressor genes may play an important role in the development and progression of solid tumors. The retinoblastoma tumor suppressor gene (Rb-1), located in chromosome 13q14, is the most extensively characterized tumor suppressor gene (Friend et al., Nature, 323: 643 (1986); Lee et al., Science, 235: 1394 (1987); Fung et al., Science, 236: 1657 (1987)]. The Rb-1 gene product, a 105 kDa nuclear phosphoprotein, apparently plays an important role in cell cycle regulation [Lee et al., supra (1987); Howe et al., PNAS (USA), 87; 5883 (1990)]. Altered or lost expression of the Rb protein is caused by inactivation of both gene alleles either through a point mutation or a chromosomal deletion. Rbi gene alterations have been found to be present not only in retinoblastomas [Friend et al., supra (1986); Lee et al., supra (1987); Fung et al., supra (1987)] but also in other malignancies such as osteosarcomas [Friend et al., supra (1986)], small cell lung cancer [Hensel et al., Cancer Res., 50: 3067 (1990); Rygaard et al., Cancer Res. 50: 5312 (1990)] and breast cancer [Lee et al., Science, 241: 218 (1988); T'Ang et al., Science, 242: 263 (1988); Varley et al., Oncogene, 4: 725 (1989)]. Restriction fragment length polymorphism (RFLP) studies have indicated that such tumor types have frequently lost heterozygosity at 13q suggesting that one of the Rb-1 gene alleles has been lost due to a gross chromosomal deletion [Bowcock et al., Am. J. Hum. Genet., 46: 12 (1990)].
The deletion of the short arm of chromosome 3 has been associated with several cancers, for example, small cell lung cancer, renal and ovarian cancers; it has been postulated that one or more putative tumor suppressor genes is or are located in the p region of chromosome 3 (ch. 3p) [Minna et al., Symposia on Quantitative Biology, Vol. LI: 843–853 (SCH Lab 1986); Cohen et al., N. Eng. J. Med. 301: 592–595 (1979); Bergerham et al., Cancer Res. 49: 13901396 (1989); Whang-Peng et al., Can. Genet. Cytogenet., II: 91–106 (1984; and Trent et al., Can. Genet. Cytogenet., 14: 153–161 (1985)].
The above-indicated collection of amplified and deleted genes is far from complete. As the Saint-Ruf et al. study (supra) of oncogene amplification in cells showing cytogenetic evidence of amplification, such as double minutes (DMs) or homogeneously staining regions (HSRs), indicated, the amplified genes were not known oncogenes in most cases. As Dutrillaux et al., supra indicated, “cytogenetic data remains scarce” for “the most frequent malignant tumors”—breast carcinomas.
Discovery of genetic changes involved in the development of solid tumors has proven difficult. Karyotyping is impeded by the low yield of high quality metaphases and the complex nature of chromosomal changes [Teyssier, J. R., Cancer Genet. Cytogenet., 31: 103 (1989)]. Although molecular genetic studies of isolated tumor DNA have been more successful and permitted detection of common regions of allelic loss, mutation or amplification [Fearon et al., Cell, 61: 759 (1990); Sato et al., Cancer Res., 50: 7184 (1990); Alitalo et al., Adv. Cancer Res., 47: 235 (1986); and Schwab and Amler, Genes Chrom. Cancer, 1: 181 (1990)], such molecular methods are highly focused, targeting one specific gene or chromosome region at a time, and leaving the majority of the genome unexamined.
Thus, a research tool leading to the identification of amplified and deleted genes and providing more cytogenetic data regarding tumors, especially tumor progression and invasiveness is needed in tumor cytogenetics. CGH provides such a molecular cytogenetic research tool.
CGH facilitates the genetic analysis of tumors in that it provides a copy number karyotype of the entire genome in a single step. Regions of tumor DNA gain and loss are mapped directly onto normal chromosomes. Comparisons of primary tumors with their metastases by CGH should be informative concerning cancer progression.
The ability to survey the whole genome in a single hybridization is a distinct advantage over allelic loss studies by restriction fragment length polymorphism (RFLP) that target only one locus at a time. RFLP is also restricted by the availability and informativeness of polymorphic probes.
The copy number karyotype determined by CGH may become as important for diagnostic and/or prognostic assessment of solid tumors as conventional karyotyping now is for hematologic malignancies. [Yunis, J. J., Science, 221: 227 (1983); Solomon et al., Science, 254: 1153 (1991).]